This invention relates to a sliding material suitable for use in the manufacture of sliding parts such as those used in automobiles, industrial machines, and construction equipment. In particular, it relates to a lead-free sliding material and a method for its manufacture.
Moving parts of automobiles, industrial equipment, and construction equipment are often slidably supported by members which will be collectively referred to as sliding parts. Examples of such sliding parts are plain bearings which support rotating shafts in automobiles, side plates for restraining the side surfaces of gears in gear pumps of hydraulic equipment, and cylinders, swash plates, and shoes of piston pumps.
When equipment having such sliding parts breaks down and becomes too expensive to repair or becomes old and can no longer be used as desired, it is usually discarded. In order to conserve natural resources, many of the materials forming the components of such equipment are recycled or recovered and reused. However, sliding parts of equipment are often incapable of being recycled and are instead disposed of by burial.
Sliding parts typically having a sliding surface comprising a Cu-based bearing alloy. Such an alloy does not have sufficient mechanical strength to form a sliding part by itself, and a sliding part formed entirely from such an alloy would be too expensive. Therefore, sliding parts usually further include a rigid backing plate, such as a steel plate, bonded to the bearing alloy layer. The backing plate provides the sliding part with mechanical strength, while the bearing alloy layer provides the sliding part with good sliding properties. The bearing alloy layer and the backing plate are metallically bonded to each other, so it is not possible to mechanically separate the bearing alloy layer from the backing plate and separately recover them. It is conceivable to melt a sliding part and recover the steel contained in the backing plate, but the resulting molten steel will contain large amounts of Pb, Cu, and Sn introduced into the steel from the bearing alloy layer. These elements are undesirable components of steel and are difficult to remove. Thus, sliding parts cannot be easily recycled, and they are usually disposed of by burial as industrial waste.
From long in the past, lead bronze (referred to for short as LBC3 and having the composition Cu-10Sn-10Pb) has been frequently used as a Cu-based bearing alloy for sliding parts. Lead bronze contains lead dispersed in a Cu—Sn alloy matrix. The hard Cu—Sn alloy matrix can support a member being slidably supported by a sliding part (referred to below as “the opposing member”) without wearing, and the lead spreads as a thin layer on the surface of the opposing member and provides good sliding properties by acting as a lubricating oil. Lead bronze is inexpensive and has good sliding properties, and for this reason it has been used in all types of sliding parts.
However, if sliding parts containing lead bronze are disposed of by burial in landfills and are contacted by so-called acid rain, the lead present in the lead bronze may be dissolved out by the acid rain and pollute underground water. If underground water containing lead is ingested by humans or livestock over a long period of time, the lead accumulates within the body and eventually cause lead poisoning. Therefore, the use of lead is now being regulated on a worldwide scale. In order to comply with such regulations, there is a strong demand in industries using sliding parts for a bearing alloy which does not contain lead.
Cu—Sn—Bi sliding alloys having Sn and Bi added to a copper base have been proposed as lead-free sliding alloys. In such sliding alloys, Bi performs the same function as Pb in conventional lead bronze, i.e., it forms a thin film covering the surface of the opposing member and acts as a lubricating oil to provide good sliding properties.
There have been a number of disclosures of bearing alloys containing Cu, Sn, and Bi. For example, JP H9-249924-A1 discloses a copper alloy and sliding bearing in which any of Ag, Sn, Sb, In, Mn, Fe, Bi, Zn, Ni, and Cr is contained in solid solution in a Cu matrix. JP H10-330868-A1 discloses a copper-based sintered alloy containing 5-50 mass % of Bi in a copper phase. JP 2003-194061-A1 discloses a copper-based sintered sliding material in which 1-11 mass % of Sn and at most 25 mass % of Bi are contained in a Cu base. JP 2005-163074-A1 discloses a Cu—Sn—Bi based sliding material in which Bi derived from an Sn—Bi alloy powder is uniformly distributed in a powder mixture, the mixed powder is dispersed on a steel plate, and then sintering is performed. In each of these publications, Bi is uniformly dispersed in a copper-based matrix.
In construction equipment and hydraulic pumps operating under a high load and at high speeds, lubricating oil is supplied to the sliding surfaces of sliding parts. A sliding part made from a conventional copper-based sliding material in which a Cu—Sn—Bi alloy is bonded to a steel plate does not exhibit a sufficient lubricating effect even when used with a lubricating oil, and of course it can not be used in a dry state without a lubricating oil.
In general, with sliding parts intended for use in a lubricated state, lubricating oil must always be present in a prescribed amount on a sliding surface of the sliding part. In order to ensure the presence of oil during operation of the sliding part, oil grooves for storing lubricating oil are normally formed in the sliding surface. Oil grooves typically have the shape of an elongated curve or a hemispherical shape. Oil grooves are usually formed in a sliding part by machining or pressing of the surface of the bearing alloy layer of the sliding material.
However, when an oil groove is formed in a bearing alloy layer, the thickness of the bearing alloy layer is reduced where the oil grooves are formed, and the reduced thickness decreases the bonding strength between the bearing alloy layer and the backing plate. As a result, in severe conditions of use, the bearing alloy layer sometimes peels off of the backing plate. If an oil groove is formed by press working, the portion of the sliding part where the oil groove is formed undergoes work hardening, which can be an impediment to subsequent working of the sliding material. For example, when the sliding material is to be formed into a cylindrical bearing, work hardening resulting from pressing can make it impossible to form the sliding material into a perfect cylinder.